1. Field of the Invention
The present invention is directed to an x-ray generator of the type having an electron source which emits electrons which follow a specified path, means for deflecting the electrons from the specified path in the direction of an anode, and a beam guidance system, including a solenoid coil, which guides the deflected electrons onto the anode.
2. Description of the Prior Art
The electron beam-shaping part of a computed tomography apparatus basically composed of an electron source, an evacuated drift tube equipped with ion traps, and a lens system which generates time-dependent, magnetic dipole and quadrupole fields and which deflects the electrons from the horizontal beam axis and focuses them onto one a number of tungsten anodes which surround the patient in the fashion of a half-ring. (See, for example, U.S. Pat. Nos. 4,352,021 and 4,521,900 and 4,625,150 and "High-speed Computed Tomography: Systems and Performance," Peschmann et al., Applied Optics, Vol. 24, No. 25, December 1985, pp. 4052-4060) A detector that is likewise shaped like a half-ring measures the intensity of the x-rays emitted in the region of the electron focus, which has a size of approximately 2.5.times.5 mm.sup.2. The x-rays are collimated into a fan-like beam by a diaphragm system and are partially absorbed in the patient according to the density of the respectively transirradiated tissue segment. The position of the x-ray source relative to the patient can be modified very quickly by deflecting the electron beam on the anode rings. The useable angular range, however, amounts to a maximum of 210.degree. due to physical limitations imposed by the structure.
Conventional tomography systems are equipped with rotating anode x-ray tubes (40 kW, 140 kV) operated pulsed, and with ring detectors, with mechanical drives moving both the x-ray tubes and the detector elements in a circle around the patient. The stability and loadability of the mechanical components, which are thus subjected to strong centrifugal forces limits the rotational frequency of the x-ray tubes to a maximum of 1 rotation/sec.
An x-ray generator known from German OS 195 15 415 is composed of an electron source, of a beam guidance deflecting the electrons onto a circular rated path, of a ring anode arranged axially offset relative to the beam guidance, and of electron-optical components that couple the electrons circulating within a solenoid coil out and deflect them in a direction of the ring anode.